Optical coupling devices using a Mach-Zehnder interferometer (hereinafter referred to as MZI) have been known (see Patent Literature 1). Displays can be expected to be more compact by using such optical coupling devices.
Patent Literature 1 discloses a technology for multiplexing or demultiplexing two visible light rays having different wavelengths through the use of a phase difference due to the difference in the optical path length in an MZI optical waveguide into which the two visible light rays enter. However, the technology in Patent Literature 1 can multiplex only two light rays having different wavelengths, and therefore cannot be applied to image forming apparatuses that need to multiplex at least three visible light rays of R, G and B.
Therefore, as shown in Patent Literature 2, the present applicant has proposed a multiplexer and an image projection apparatus that make it possible to project an image in the image projection apparatus by multiplexing three visible light rays of R, G and B, which are the three primary colors of light.
FIGS. 10A and 10B are diagrams illustrating the multiplexer that has been disclosed by the present applicant in Patent Literature 2. FIG. 10A is a plan diagram, and FIG. 10B is a side diagram showing the multiplexer, where the incident ports of the waveguides are exposed.
A multiplexer 10′ is provided with a substrate 210 formed of silicon (Si), a BOX layer 215 formed of silicon dioxide (SiO2) on the substrate 210, a cover layer 220 formed of SiO2 on the BOX layer 215, and a first waveguide 101, a second waveguide 102′ and a third waveguide 103′ that are formed in the cover layer 220 and placed within a plane parallel to the upper surface of the BOX layer 215.
Here, the substrate 210, the BOX layer 215 and the cover layer 220 form the main body 100 of the multiplexer 10.
A red light ray (R) having a wavelength λR of 620 to 750 nm, a green light ray (G) having a wavelength λG of 495 to 570 nm, and a blue light ray (B) having a wavelength λB of 450 to 495 nm, which are in a single mode having different wavelengths, enter into the first waveguide 101, the second waveguide 102′ and the third waveguide 103′ through the incident ports 101a′, 102a′ and 103a′ that are exposed from one end face of the BOX layer 215 and the cover layer 220. The RGB light rays respectively propagating through the first waveguide 101, the second waveguide 102′ and the third waveguide 103′ are multiplexed and emitted from the other end 102b′ of the second waveguide 102′ that is exposed from the other end face of the clad layer 220.
The first waveguide 101, the second waveguide 102′ and the third waveguide 103′ are placed at intervals that can prevent stray light from being generated. A first multiplexing unit 110, a second multiplexing unit 120 and a third multiplexing unit 130 are provided along the path of the second waveguide 102′ through which visible light propagates starting from the incident port 102a′ side. The first multiplexing unit 110, the second multiplexing unit 120 and the third multiplexing unit 130 are formed as directional couplers in such a manner that the third waveguide 103′ makes contact with the second waveguide 102′ in the first multiplexing unit 110 and in the third multiplexing unit 130, and the first waveguide 101′ makes contact with the second waveguide 102′ in the second multiplexing unit 120 so that the visible light rays R, G and B can be multiplexed.